1. Technical Field of the Invention
This invention relates to radio telecommunication systems and, more particularly, to a system and method of automatically conveying a Wireless Office System (WOS) frequency set to mobile stations (MSs) belonging to the WOS whenever the WOS changes its frequency set.
2. Description of Related Art
Multi-level layered cellular radio architectures are known in the art and are utilized to increase capacity in the cellular network, especially in densely populated urban areas. In these hierarchical networks, microcells and picocells share capacity in areas also covered by larger macrocells. Server selection and handoff decisions can be quite complex in these networks, and U.S. Pat. Nos. 5,499,386 and 5,640,677 to Karlsson disclose a method of best server selection in which a decision as to the best serving cell for a mobile station is made based upon a preference value assigned to each cell and the signal strength with the mobile station. Microcells and picocells are preferred in this method since they add capacity to the network. For purposes of this disclosure, the term. "microcell" is construed as including picocells as well.
In an office building, the owner may install a private wireless office system (WOS) which may include a plurality of microcells providing cellular coverage for private subscribers within the office building. Nearby, there may be a base station for a macrocell which provides public coverage of an area which includes the office building. Signals from outside the office building (for example, from the nearby macrocell base station) penetrate the building, and it is possible for a MS within the building to access the Public Land Mobile Network (PLMN) through the macrocell. In fact, as shown in FIG. 1, the signal strength from the public macrocell 11 often exceeds the signal strength from the private WOS microcell 12. MSs typically scan for the channel with the strongest signal strength, and then access their network on that channel. It is desirable, however, for MSs that belong to the private system to access the WOS through the private microcell rather than accessing the P through the public macrocell. This may be accomplished by programming the MSs that belong to the private system with a Public Service Profile/Private Operating Frequency (PSP/POF). MSs first access the public system and determine if there is a PSP match. If so, the Ms utilize the corresponding POF to access the private WOS. The MSs may be programmed with the PSP/POF utilizing the Over-the-Air Activation System (OATS) which utilizes the IS-136 R-DATA message to program the MS over the air interface. Therefore, when the MSs attempt to access wireless service from within the building, they automatically select one of the microcells within the private system. The difference from the regular hierarchical cell structure described by Karlsson is that the private microcell is preferred only for the subscribers of that private system.
Portions of the frequency spectrum are allocated to various cellular operators. Each operator implements a frequency reuse plan which is designed to provide opium radio coverage within the operator's service area while minimizing co-channel interference. Once the operator has implemented the frequency plan, then any private WOSs within the service area can obtain frequencies for use in their private microcells. In a process much like a MS scanning for the best server, each WOS scans the entire cellular spectrum and picks a few operating frequencies that provide the least interference based on signal strength measurements taken across the spectrum. The best channel is picked for the digital control channel (DCCH). Then, as long as radio conditions are static, and the public operator does not retune (i.e., change the frequency plan for the public system), the frequencies for the WOS remain the same.
However, the radio conditions do not stay static for long. With growth in the number of subscribers, expansion of the network, and shifting traffic densities in the public system, cells are often moved, and new cells are added, and the public operator must retune the system. With. adaptive channel allocation (ACA) implemented, it is likely that frequencies will change even more often. This creates a problem for the WOSs within the operator'S service area. When the public system retunes, the WOSs automatically retune also, in order to pick operating frequencies that provide the least interference. When a WOS retunes, however, the MSs belonging to the WOS are no longer programmed with the proper WOS frequencies. In atypical scenario, a subscriber may leave his office on Friday and take his MS home with him. Over the weekend, the public system retunes, prompting the subscribers WOS to retune as well. When the subscriber returns to his office on Monday and originates a call, his MS cannot locate a WOS frequency, and accesses the public system instead.
Typically, the owner of the private WOS may pay a monthly flat fee to the public operator for system access, or may pay a preferential rate for calls within the WOS. If a subscriber belonging to the WOS inadvertently accesses the public system rather than the WOS, then the private subscriber's call is charged for the call at the public rate rather than the private system rate. If so, the subscriber may think his calls are free, or at the preferential rate, when they are not.
There are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed herein. One solution is to manually reprogram all the MSs in the WOS whenever the WOS retunes. However, this is an extremely inefficient and difficult process when there are large numbers of MSs in the private system. In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method of automatically conveying a WOS frequency set to MSs belonging to the WOS whenever the WOS changes its frequency set. The present invention provides such a system and method.